Nowadays, an optical electronics technology has been used in various fields such as an information input/output technology as represented by a compact disk and a photonic technology using an optical fiber. As a device supporting the optical electronics technology, various kinds of semiconductor laser diodes or semiconductor optical amplifiers have been developed.
Regarding the semiconductor laser diode, various structures have been developed. A waveguide type semiconductor laser diode is an example of the structures. In general, a waveguide configuration of the waveguide type semiconductor laser diode is designed to meet a fundamental mode condition in the case where information signals are transmitted in the form of light energy or the laser is used as an excitation light source of the optical fiber amplifier. The reason the fundamental mode condition is set is to overcome the following problems. That is, in general, under a multimode condition, there arise a problem that multimode dispersion exerts influence and a problem that signal light cannot be efficiently connected when the signal light is connected to another optical waveguide such as an optical fiber or a lens.
Up to now, in the waveguide type semiconductor laser diodes, a waveguide that allows transmission of only fundamental mode has been used to obtain a fundamental mode light. However, the waveguide that allows transmission of only fundamental mode has a problem that an amount of current supplied to the semiconductor laser diode is limited because of its small waveguide width (about 2 to 4 μm). This problem imposes technical limitations to gain a high optical output.
To that end, the applicants of the present invention proposed a semiconductor laser diode that can output a fundamental mode light and attain high optical output (Patent Documents 1 and 2). The semiconductor laser diode of Patent Documents 1 and 2 is configured such that fundamental mode waveguides are connected on both sides of a 1-input/N-output (N is a positive integer) type multimode interference waveguide (hereinafter abbreviated as “1×N-MMI (Multi mode Interference”) semiconductor laser diode (see Non-Patent Document 1). As compared with the conventional waveguide that allows transmission of only fundamental mode, a waveguide width can be increased to enable high optical output.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 11-68241
[Patent Document 2]
Japanese Unexamined Patent Publication No. 11-68242
[Non-Patent Document 1]
Lucas B. Soldano, Journal of Lightware Technology, Vol. 13, No. 4, pp. 615-627, 1995